What is about the first column in R's dataset mtcars?

Question

I think I am missing a fundamental concept about R\'s data frames.

head(mtcars)
                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX         


        

Answer 1

They are row names, to access them use:

rownames(mtcars)

For column names use colnames, to see both row and column names, we can use:

dimnames(mtcars)

To modify, for example the first row:

rownames(mtcars)[1] <- "myNewName"

When data frame is created with data.frame, row names are assigned with 1:n numbers.

mydata <- data.frame(x = 1:5)

Then we can modify them:

rownames(mydata) <- paste0("MyName", 1:5)

Or we can add rownames when creating the data.frame:

mydata <- data.frame(x = 1:5, row.names = paste0("MyName", 1:5))

Note: rownames are not very reliable, for example see this post. (this could be subjective opinion and I avoid them by reassigning rownames to columns)

data.table and dplyr packages prefer not to have them. You can always reassign rownames into a columns as:

mydata$myNames <- rownames(mydata)